(a) Field of the Invention
The present invention relates to nonflammable electrolyte compositions and lithium secondary batteries thereof, more particularly to nonflammable electrolyte compositions comprising carbonate-, thiocarbonate-, and phosphate-based solvent and inorganic additives which are capable of forming a solid electrolyte interface (hereinafter, referred to as `SEI`) on a surface of a negative electrode, thereby greatly enhancing the overall safety of the battery, and lithium secondary batteries thereof.
(b) Description of the Related Art
With the recent proliferation in the use of portable electronic devices, coupled with advancements made enabling increasingly smaller sizes and weights for these devices, research is being actively pursued to improve the energy density capabilities of lithium secondary batteries. These portable electronic machines have made it necessary to develop lithium secondary batteries with high energy density.
In the past, although lithium metal was used as the anode active material in lithium secondary batteries, a serious problem of dendritic formation on the surface of the lithium metal resulted during charging and discharging. This may cause short circuits, or more seriously, it could lead to the explosion of the battery. To prevent such problems, carbonaceous material is now widely used for the negative active material.
For cathode active materials in secondary batteries, metal chalcogenide compounds, enabling insertion and separation of lithium ions, are generally used, i.e. composite metal oxides such as LiMn.sub.2 O.sub.4, LiMnO.sub.2, LiCoO.sub.2, LiNiO.sub.2, and LiNi.sub.1-x Co.sub.x O.sub.2 (0&lt;x &lt;1). The Mn-based active materials, LiMn.sub.2 O.sub.4, and LiMnO.sub.2, can be easily synthesized, are less expensive than the other materials, and have minimal negative affects on the environment. However, the capacities of these materials are low. In particular, LiMn.sub.2 O.sub.4 has a relatively low discharge capacity as compared to other lithiated transition metal oxides such as LiCoO.sub.2 and LiNiO.sub.2. Furthermore, when cycled at high rates of charge and discharge are cycled, the discharge capacity is excessively reduced. When the charge and discharge operations are continuously performed at high temperatures, manganese ions readily elute into the electrolyte from the surface of LiMn.sub.2 O.sub.4. These manganese ions seriously deteriorate the cycle life characteristics of the battery. LiCoO.sub.2 has been commercialized by Sony Energy Tec. as it exhibits an electrical conductivity at room temperature, provides a high level of battery voltage, and has exceptional electrode characteristics, even though it is unsafe when charging or discharging at high rates, and is more costly than the other materials. LiNiO.sub.2 has a high discharge and charge capacity and is the least expensive of the above active materials for positive electrodes, but is not easily synthesized.
In addition, the choice of suitable electrolytes is one of the factors for improving cell characteristics because reactions of electrodes and electrolyte have an effect on cell performance. The electrolyte systems have previously only played a role of transfer of lithium ions. However, these electrolytes can be decomposed, causing severe deterioration of cell performance. Therefore, carbonate-based solvents are known preferable as solvents of high-voltage lithium secondary batteries. Unfortunately, carbonate-based organic solvents can be easily ignited by exterior ignition sources. In this regard, various methods have been developed to advance the safety of lithium secondary batteries containing organic solvents.
In one approach, Sony Energy Tec. tried to apply a mixture of phosphate-based and carbonate-based solvents as electrolyte solvent to improve cell safety. However, this method has disadvantages of decaying cell performance by increasing non-reversible capacity and decreasing of cell life characteristics due to the decomposition of phosphate during the charge and discharge reactions.
As a second approach, U.S. Pat. No. 5,830,600 discloses electrolyte containing a phosphate, a phospholane, a cyclophosphazene, a silane, a fluorinated carbonate, a fluorinated polyether, or mixtures thereof. However, this method also results in a decline of cell performance due to the decomposition of phosphate-based electrolyte solvent.